Inverse shape replication method of making ceramic composite articles

ABSTRACT

A method of producing a self-supporting ceramic composite body having therein at least one cavity which inversely replicates the geometry of a positive mold of parent metal. The method includes embedding the mold of parent metal within a conformable bed of filler to provide therein a cavity shaped and filled by the mold. The assembly is heated to melt the parent metal mold, e.g., an aluminum parent metal mold, and contacted with an oxidant to oxidize the molten parent metal to form a polycrystalline material which grows through the surrounding bed of filler, the molten metal being drawn through the growing polycrystalline material to be oxidized at the interface between the oxidant and previously formed oxidation reaction product whereby the cavity formerly filled by the mold of parent metal is eventually evacuated of the metal. There remains behind a cavity whose shape inversely replicates the original shape of the mold. The method provides ceramic composite articles having therein at least one cavity inversely replicating the shape of the mold which supplied the parent metal for oxidation.

This is a continuation of application(s) Ser. No. 07/983,191 filed onNov. 30, 1992, which issued on Jan. 4, 1994, as U.S. Pat. No. 5,275,987,which is a Continuation of U.S. Pat. No. 5,168,081, which issued on Dec.1, 1992 from U.S. application Ser. No. 07/763,681, filed Sep. 23, 1991which is a Continuation of U.S. Ser. No. 07/329,794, filed on Mar. 28,1989, which issued Sep. 24, 1991, as U.S. Pat. No. 5,051,382, which is aDivisional of U.S. Ser. No. 06/823,542, which filed Jan. 27, 1986, whichissued May 9, 1989 as U.S. Pat. No. 4,828,785.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention broadly relates to ceramic composite bodies havingone or more shaped cavities therein and to methods of making the same.In particular, the invention relates to ceramic composite bodiescomprising a polycrystalline matrix embedding a filler and having atleast one cavity of selected geometry formed therein, and to methods ofmaking the composites by infiltrating a bed of filler with the oxidationreaction product of a parent metal preshaped as a positive mold which isinversely replicated to form the cavity of the ceramic composite.

2. Description of Commonly Owned Patent Applications

The subject matter of this application is related to that of copendingand Commonly Owned U.S. Pat. No. 4,851,375, which issued on Jul. 25,1989, from U.S. patent applications Ser. No. 819,397, filed Jan. 17,1986, which was a continuation-in-part of Ser. No. 697,876, filed Feb.4, 1985, both in the names of Marc S. Newkirk et al and entitled"Composite Ceramic Articles and Methods of Making Same." These copendingapplications and patents disclose a novel method for producing aself-supporting ceramic composite by growing an oxidation reactionproduct from a parent metal into a permeable mass of filler. Theresulting composite, however, has no defined or predeterminedconfiguration.

The method of growing a ceramic product by an oxidation reaction isdisclosed generically in Commonly Owned U.S. Pat. No. 4,713,360 whichissued on Dec. 15, 1987 and was based on U.S. applications Ser. No.818,943, filed Jan. 15, 1986 as a continuation-in-part of Ser. No.776,964, filed Sep. 17, 1985, which was a continuation-in-part of Ser.No. 705,787, filed Feb. 26, 1985, which was a continuation-in-part ofSer. No. 591,392, filed Mar. 16, 1984, all in the names of Marc S.Newkirk et al and entitled "Novel Ceramic Materials and Methods ofMaking The Same." The employment of an unusual oxidation phenomenon asdescribed in the aforesaid Commonly Owned patent applications andpatent, which may be enhanced by the use of an alloyed dopant, affordsself-supporting ceramic bodies grown as the oxidation reaction productfrom a precursor parent metal and a method of making the same. Themethod was improved upon by the use of external dopants applied to thesurface of the precursor parent metal as disclosed in Commonly OwnedU.S. Pat. No. 4,853,352, which issued on Aug. 1, 1989, from U.S. patentapplication Ser. No. 220,935, filed on Jun. 23, 1988, as a continuationof U.S. applications Ser. No. 822,999, filed Jan. 27, 1986, which was acontinuation-in-part of Ser. No. 776,965, filed Sep. 17, 1985, which wasa continuation-in-part of Ser. No. 747,788, filed Jun. 25, 1985, whichwas a continuation-in-part of Ser. No. 632,636, filed Jul. 20, 1984, allin the names of Marc S. Newkirk et al and entitled "Methods of MakingSelf-Supporting Ceramic Materials".

The entire disclosures of each of the foregoing Commonly Owned patentapplications and patents are expressly incorporated herein by reference.

BACKGROUND AND PRIOR ART

In recent years, there has been increasing interest in the use ofceramics for structural applications historically served by metals. Theimpetus for this interest has been the superiority of ceramics withrespect to certain properties, such as corrosion resistance, hardness,modulus of elasticity, and refractory capabilities when compared withmetals. However, there remains a major requirement for improved strengthunder tensile loading, greater damage tolerance (toughness) and improvedperformance reliability if ceramic components are to enjoy fullcommercial success.

Current efforts at producing higher strength, more reliable, and tougherceramic articles are largely focused upon (1) the development ofimproved processing methods for monolithic ceramics and (2) thedevelopment of new material compositions, notably ceramic matrixcomposites. A composite structure is one which comprises a heterogeneousmaterial, body or article made of two or more different materials whichare intimately combined in order to attain desired properties of thecomposite. For example, two different materials may be intimatelycombined by embedding one in a matrix of the other. A ceramic matrixcomposite structure typically comprises a ceramic matrix which enclosesone or more diverse kinds of filler materials such as particulates,fibers, rods or the like.

The traditional methods of preparing ceramic articles involve thefollowing general steps: (1) Preparation of matrix material in powderform. (2) Grinding or milling of powders to obtain very fine particles.(3) Formation of the powders into a body having the desired geometry(with allowance for shrinkage during subsequent processing). Forexample, this step might be accomplished by uniaxial pressing, isostaticpressing, injection molding, tape casting, slip casting or any ofseveral other techniques. (4) Denslfication of the body by heating it toan elevated temperature such that the individual powder particles mergetogether to form a coherent structure. Preferably, this step isaccomplished without the application of pressure (i.e., by pressurelesssintering), although in some cases an additional driving force isrequired and can be provided through the application of externalpressure either uniaxially (i.e., hot pressing) or isostatically, i.e.,hot isostatic pressing. (5) Finishing, frequently by diamond grinding,as required.

A considerable amount of current work is directed toward improved powderprocessing technologies, and although these efforts have resulted inimprovements in ceramic performance, they are also complicated andgenerally less than cost-effective. The emphasis in such technologieshas been in two areas: (1) improved methods of producing ultrafine,uniform powder materials using sol-gel, plasma and laser techniques, and(2) improved methods of densification and compaction, including superiortechniques for sintering, hot pressing and hot isostatic pressing. Theobject of these efforts is to produce dense, fine-grained, flaw-freemicrostructures, and, in fact, some improvements in performancecapabilities in ceramics have been attained in some areas. However,these developments tend to result in dramatic increases in the cost ofproducing ceramic structures. Thus, cost becomes a major restriction onthe commercial application of ceramics.

Another limitation in ceramic engineering which is aggravated by modernceramic processing is scaling versatility. Conventional processes aimedat densification (i.e., removal of voids between powder particles) areincompatible with large one-piece structural application possibilitiesfor ceramics. An increase in article size presents several problemsincluding, for example, increased process residence times, stringentrequirements for uniform process conditions over a large process volume,cracking of parts due to nonuniform densification or thermally inducedstresses, warping and sagging of parts during sintering, excessivecompaction forces and die dimensions if hot pressing is used, andexcessive pressure vessel costs due to internal volume and wallthickness requirements in the case of hot isostatic pressing.

When these traditional methods are applied to the preparation of ceramicmatrix composite materials, additional difficulties arise. Perhaps themost serious problems concern the densification step, number (4) above.The normally preferred method, pressureless sintering, can be difficultor impossible with particulate composites if the materials are nothighly compatible. More importantly, normal sintering is impossible inmost cases involving fiber composites even when the materials arecompatible, because the merging together of the particles is inhibitedby the fibers which tend to prevent the necessary displacements of thedensifying powder particles. These difficulties have been, in somecases, partially overcome by forcing the densification process throughthe application of external pressure at high temperature. However, suchprocedures can generate many problems, including breaking or damaging ofthe reinforcing fibers by the external forces applied, limitedcapability to produce complex shapes (especially in the case of uniaxialhot pressing), and generally high costs resulting from low processproductivity and the extensive finishing operations sometimes required.

Additional difficulties can also arise in the blending of powders withwhiskers or fibers and in the body formation step, number (3) above,where it is important to maintain a uniform distribution of thecomposite second phase within the matrix. For example, in thepreparation of a whisker-reinforced ceramic composite, the powder andwhisker flow processes involved in the mixing procedure and in theformation of the body can result in non-uniformities and undesiredorientations of the reinforcing whiskers, with a consequent loss inperformance characteristics.

The Commonly Owned patents and applications describe new processes whichresolve some of these problems of traditional ceramic technology asdescribed more fully therein. The present invention combines theseprocesses with additional novel concepts to remove a further limitationof ceramic technology, namely the formation of complex structures to netor near net shape, and more particularly the difficulties in formationof shapes having complicated internal cavities and especially shapeshaving re-entrant cavities. With such shapes, the ceramic body formationmethods (step (3) above) which one would normally use are notapplicable, because the internal mold required to establish the desiredpart geometry can not be removed after the body is formed around it.While such part geometries can be prepared by grinding the desired shapefrom a finished ceramic blank, this approach is rarely used because ofthe prohibitive costs of ceramic grinding.

The present invention provides for fabrication of ceramic composites ofcertain predetermined interior geometry by an unusual oxidationphenomenon which overcomes the difficulties and limitations associatedwith known processes. This method provides shaped cavity-containingceramic bodies typically of high strength and fracture toughness by amechanism which is more direct, more versatile and less expensive thanconventional approaches.

The present invention also provides means for reliably producing ceramicbodies having shaped cavities therein of a size and thickness which aredifficult or impossible to duplicate with the presently availabletechnology.

SUMMARY OP THE INVENTION

In accordance with the present invention, there is provided a method forproducing a self-supporting ceramic composite body having therein atleast one cavity which inversely replicates the geometry of a positivepattern or mold (hereafter "mold"). The ceramic composite comprises aceramic matrix having a filler embedded therein, the matrix beingobtained by oxidation of a parent metal to form a polycrystallinematerial which consists essentially of the oxidation reaction product ofsaid parent metal with an oxidant, e.g., with a vapor-phase oxidant,and, optionally, one or more non-oxidized constituents of the parentmetal. The method comprises the following steps: the parent metal isshaped to provide a mold, and then is embedded within a conformablefiller which inversely replicates the geometry of the shaped parentmetal. The filler (1) is permeable to the oxidant when required as inthe case where the oxidant is a vapor-phase oxidant and, in any case, ispermeable to infiltration by the developing oxidation reaction product;(2) has sufficient conformability over the heat-up temperature intervalto accommodate the differential thermal expansion between the filler andthe parent metal plus the melting-point volume change of the metal; and(3) at least in a support zone thereof enveloping the mold, isintrinsically self-bonding only at a temperature which is above themelting point of said parent metal but below and preferably very closeto the oxidation reaction temperature, whereby said filler hassufficient cohesive strength to retain the inversely replicated geometrywithin the bed upon migration of the parent metal as described below.The embedded shaped parent metal is heated to a temperature region aboveits melting point but below the melting point of the oxidation reactionproduct to form a body of molten parent metal, and the molten parentmetal is reacted in that temperature region or interval with the oxidantto form the oxidation reaction product. At least a portion of theoxidation reaction product is maintained in that temperature region andin contact with and between the body of molten metal and the oxidant,whereby molten metal is progressively drawn from the body of moltenmetal through the oxidation reaction product, concurrently forming thecavity as oxidation reaction product continues to form within the bed offiller at the interface between the oxidant and previously formedoxidation reaction product. This reaction is continued in thattemperature region for a time sufficient to at least partially embed thefiller within the oxidation reaction product by growth of the latter toform the composite body having the aforesaid cavity therein. Finally,the resulting self-supporting composite body is separated from excessfiller, if any.

In another aspect of the invention, there is provided a self-supportingceramic composite body having therein a cavity which inverselyreplicates the shape or geometry of a mold of a parent metal precursorand comprises a ceramic matrix having filler incorporated therein. Thematrix consists essentially of a polycrystalline oxidation reactionproduct having interconnected crystallites formed upon oxidation of theparent metal precursor, and optionally a metallic constituent or pores,or both, as described above.

The materials of this invention can be grown with substantially uniformproperties throughout their cross section to a thickness heretoforedifficult to achieve by conventional processes for producing denseceramic structures. The process which yields these materials alsoobviates the high costs associated with conventional ceramic productionmethods, including fine, high purity, uniform powder preparation, greenbody forming, binder burnout, sintering, hot pressing and hot isostaticpressing. The products of the present invention are adaptable orfabricated for use as articles of commerce which, as used herein, isintended to include, without limitation, industrial, structural andtechnical ceramic bodies for such applications where electrical, wear,thermal, structural or other features or properties are important orbeneficial, and is not intended to include recycled or waste materialssuch as might be produced as unwanted by-products in the processing ofmolten metals.

As used in this specification and the appended claims, the terms beloware defined as follows:

"Ceramic" is not to be unduly construed as being limited to a ceramicbody in the classical sense, that is, in the sense that it consistsentirely of non-metallic and inorganic materials, but rather refers to abody which is predominantly ceramic with respect to either compositionor dominant properties, although the body may contain minor orsubstantial amounts of one or more metallic constituents derived fromthe parent metal, or reduced from the oxidant or a dopant, mosttypically within a range of from about 1-40% by volume, but may includestill more metal.

"Oxidation reaction product" generally means one or more metals in anyoxidized state wherein a metal has given up electrons to or sharedelectrons with another element, compound, or combination thereof.Accordingly, an "oxidation reaction product" under this definitionincludes the product of reaction of one or more metals with an oxidantsuch as those described in this application.

"Oxidant" means one or more suitable electron acceptors or electronsharers and may be a solid, a liquid or a gas (vapor) or somecombination of these (e.g., a solid and a gas) at the processconditions.

"Parent metal" as used in this specification and the appended claimsrefers to that metal, e.g., aluminum, which is the precursor for thepolycrystalline oxidation reaction product, and includes that metal as arelatively pure metal, a commercially available metal with impuritiesand/or alloying constituents, or an alloy in which that metal precursoris the major constituent; and when a specified metal is mentioned as theparent metal, e.g., aluminums, the metal identified should be read withthis definition in mind unless indicated otherwise by the context.

"Cavity" has its usual broad meaning of an unfilled space within a massor body, is not limited to any specific configuration of the space, andincludes both closed and open spaces. That is, it includes cavitieswhich are entirely closed off from communication to the exterior of themass or body containing the cavity, such as a cavity defining theinterior of a closed, hollow body. The defined term also includescavities which are open to such communication, e.g., by one or morepassageways or openings leading to the exterior of the mass or bodycontaining the cavity, and cavities which are themselves passageways oropenings. The latter type of cavity includes, for example, a simple borethrough, with openings at each end of, a cylindrical body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, cross-sectional view in elevation showing anassembly of a mold of shaped parent metal embedded within a bed ofparticulate filler and confined within a refractory vessel;

FIG. 2 is a perspective view on a slightly enlarged scale of the mold ofshaped parent metal utilized in the assembly of FIG. 1;

FIG. 3 is a plan view, partly in cross-section, of a self-supportingceramic composite body made in accordance with the invention;

FIG. 4 is a photograph of a ceramic composite prepared in accordancewith Example 1, and sectioned to show the internal geometry replicatingthe shape of a threaded rod as the parent metal; and

FIG. 5 is a photograph of a ceramic composite with the top and bottomremoved to show the shape replication of a threaded metal ingot.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

In the practice of the present invention, the parent metal is providedin the form of a mold, the geometry of which is to be inverselyreplicated as a cavity within the finished ceramic composite. Byfollowing the practices of the present invention, complex shapes can beinversely replicated within the finished ceramic composite duringformation or growth of the ceramic, rather than by shaping or machininga ceramic body. The term "inversely replicated" means that the cavity inthe ceramic composite attained by the invention process is defined byinterior surfaces of the ceramic composite which are congruent to theshape of the mold of parent metal. The mold of parent metal may besuitably shaped by any appropriate means; for example, a piece of metalsuch as a bar, billet or ingot may be suitably machined, cast, molded,extruded or otherwise shaped to provide the shaped mold. The parentmetal as the mold may have grooves, bores, recesses, lands, bosses,flanges, studs, screw threads and the like formed therein as well ashaving collars, bushings, discs, bars, or the like assembled thereto toprovide molds of virtually any desired configuration. The parent metalmold may comprise one or more unitary pieces of metal suitably shaped sothat when embedded within a conformable bed of filler, the mold definesa shaped cavity within the bed and occupies the cavity within the massof filler. When the parent metal occupying the cavity is ultimatelymelted and migrates out of the filled cavity, a shaped cavity developsin the resulting ceramic composite body. Thus, in one aspect, thepresent invention provides the advantage of making the cavity shape bymachining or forming a metal, rather than grinding or machining aceramic, which is a much more difficult and costly process.

Although the invention is described below in detail with specificreference to aluminum as the preferred parent metal, other suitableparent metals which meet the criteria of the present invention include,but are not limited to, silicon, titanium, tin, zirconium and hafnium.

A solid, liquid or vapor-phase oxidant, or a combination of suchoxidants, may be employed, as noted above. For example, typical oxidantsinclude, without limitation, oxygen, nitrogen, a halogen, sulphur,phosphorus, arsenic, carbon, boron, selenium, tellurium, and compoundsand combinations thereof, for example, silica (as a source of oxygen),methane, ethane, propane, acetylene, ethylene, and propylene (as asource of carbon), and mixtures such as air, H₂ /H₂ O and CO/CO₂, thelatter two (i.e., H₂ /H₂ O and CO/CO₂) being useful in reducing theoxygen activity of the environment.

Although any suitable oxidants may be employed, specific embodiments ofthe invention are described below with reference to use of vapor-phaseoxidants. If a gas or vapor oxidant, i.e., a vapor-phase oxidant, isused the filler is permeable to the vapor-phase oxidant so that uponexposure of the bed of filler to the oxidant, the vapor-phase oxidantpermeates the bed of filler to contact the molten parent metal therein.The term "vapor-phase oxidant" means a vaporized or normally gaseousmaterial which provides an oxidizing atmosphere. For example, oxygen orgas mixtures containing oxygen (including air) are preferred vapor-phaseoxidants, as in the case where aluminum is the parent metal, with airusually being more preferred for obvious reasons of economy. When anoxidant is identified as containing or comprising a particular gas orvapor, this means an oxidant in which the identified gas or vapor is thesole, predominant or at least a significant oxidizer of the parent metalunder the conditions obtaining in the oxidizing environment utilized.For example, although the major constituent of air is nitrogen, theoxygen content of air is the sole or predominant oxidizer for the parentmetal because oxygen is a significantly stronger oxidant than nitrogen.Air therefore falls within the definition of an "oxygen-containing gas"oxidant but not within the definition of a "nitrogen-containing gas"oxidant. An example of a "nitrogen-containing gas" oxidant as usedherein and in the claims is "forming-gas", which contains 96 volumepercent nitrogen and 4 volume percent hydrogen.

When a solid oxidant is employed, it is usually dispersed through theentire bed of filler or through a portion of the bed adjacent the parentmetal, in the form of particulates admixed with the filler, or perhapsas coatings on the filler particles. Any suitable solid oxidant may beemployed including elements, such as boron or carbon, or reduciblecompounds, such as silicon dioxide or certain borides of lowerthermodynamic stability than the boride reaction product of the parentmetal. For example, when a boron or a reducible boride is used as asolid oxidant for an aluminum parent metal the resulting oxidationreaction product is aluminum boride.

In some instances, the oxidation reaction may proceed so rapidly with asolid oxidant that the oxidation reaction product tends to fuse due tothe exothermic nature of the process. This occurrence can degrade themicrostructural uniformity of the ceramic body. This rapid exothermicreaction can be avoided by mixing into the composition relatively inertfillers which exhibit low reactivity. Such fillers absorb the heat ofreaction to minimize any thermal runaway effect. An example of such asuitable inert filler is one which is identical to the intendedoxidation reaction product.

If a liquid oxidant is employed, the entire bed of filler or a portionthereof adjacent the molten metal is coated or soaked as by immersion inthe oxidant to impregnate the filler. Reference to a liquid oxidantmeans one which is a liquid under the oxidation reaction conditions andso a liquid oxidant may have a solid precursor, such as a salt, which ismolten at the oxidation reaction conditions. Alternatively, the liquidoxidant may be a liquid, precursor, e.g., a solution of a material,which is used to impregnate part or all of the filler and which ismelted or decomposed at the oxidation reaction conditions to provide asuitable oxidant moiety. Examples of liquid oxidants as herein definedinclude low melting glasses.

The conformable filler utilized in the practice of the invention may beone or more of a wide variety of materials suitable for the purpose. Asused herein and in the claims, the term "conformable" as applied to thefiller means that the filler is one which can be packed around, laid upagainst, or wound around a mold and will conform to the geometry of themold embedded within the filler. For example, if the filler comprisesparticulate material such as fine grains of a refractory metal oxide,the mold is embedded by the filler so that the mold defines a filledcavity (filled or occupied by the mold). However, it is not necessarythat the filler be in fine particulate form. For example, the filler maycomprise wire, fibers or whiskers, or such materials as metal wool. Thefiller also may comprise either a heterogeneous or homogeneouscombination of two or more such components or geometric configurations,e.g., a combination of small particulate grains and whiskers. It isnecessary only that the physical configuration of the filler be such asto permit the mold of parent metal to be embedded by or within a mass ofthe filler with the filler closely conforming to the surfaces of themold. The parent metal mold is referred to herein and in the claims as a"mold" because the cavity ultimately formed in the composite is thenegative of the geometry of the mold. The mold thus initially forms a(filled) cavity within the bed of conformable filler, the cavity beinginitially shaped and filled by the mold.

The conformable filler useful in the practice of the invention is onewhich, under the oxidation reaction conditions of the invention asdescribed below, is permeable when the oxidant is a vapor-phase oxidant,to passage therethrough of the oxidant. In any case, the filler also ispermeable to the growth or development therethrough of oxidationreaction product. The filler also has at the temperature at which theoxidation reaction is conducted, sufficient cohesive strength formed ordeveloped initially or rapidly, so as to retain the geometry inverselyreplicated therein by conformance of the filler to the mold as moltenparent metal of the mold migrates from the cavity initially filled bythe mold, to concurrently (with the migration) form the cavity. Duringthe oxidation reaction, it appears that molten parent metal migratesthrough the oxidation reaction product being formed to sustain thereaction. This oxidation reaction product is generally impermeable tothe surrounding atmosphere and therefore the furnace atmosphere, e.g.,air, can not enter the developing cavity. In this manner, a low pressureregion develops within the cavity being formed by migration of themolten parent metal. The developing skin of oxidation reaction productis usually initially too weak to support the pressure differential thusdeveloping across it, combined with gravity forces, so that,unsupported, it tends to collapse inwardly, filling at least a part ofthe areas evacuated by the molten parent metal, and thereby losing theshape of the cavity established initially by the mold. Further, in caseswhere a vapor-phase oxidant is employed, collapse of the cavity tends toexpose the parent metal liquid surface level within the cavity to theoxidant so as to create a newly defined outer surface within theoriginal cavity which itself commences the oxidation and cavityformation process, thus completely losing the original desired shapefidelity of the developing ceramic composite body. It is even possiblefor this sequence to repeat numerous times, creating a misshapen bodycontaining an internal superstructure within its cavity, bearing littleor no resemblance to the original shape of the mold of parent metal. Inorder to avoid this loss of geometry, a filler is selected which, at atemperature above the melting point of the parent metal and close to(but below) the oxidation reaction temperature, partially sinters orotherwise bonds to itself and to the growing layer of oxidation reactionproduct sufficiently to provide structural strength from the outside ofthe cavity to retain the replicated geometry of the mold in thedeveloping cavity at least until the growing oxidation reaction productstructure attains sufficient thickness to be self-supporting against thedeveloped pressure differential across the cavity wall.

A suitable self-bonding filler is one which, at the appropriatetemperature, either intrinsically sinters or can be made to sinter orbond by appropriate additives or surface modifications of the filler.For example, a suitable filler for use with an aluminum parent metalutilizing an air oxidant comprises alumina powder with an added silicabonding agent as fine particles or castings on the alumina powder. Suchmixtures of materials will partially sinter or bond at or below theoxidation reaction conditions under which the ceramic matrix will form.Without the silica additive, the alumina particles require substantiallyhigher temperatures for bonding. Another suitable class of fillersincludes particles or fibers which, under the oxidation reactionconditions of the process, form a reaction product skin on theirsurfaces which tends to bond the particles in the desired temperaturerange. An example of this class of filler in the case where aluminum isemployed as the parent metal and air as the oxidant, is fine siliconcarbide particles (e.g., 500 mesh and finer), which forms a silicondioxide skin bonding themselves together in the appropriate temperaturerange for the aluminum oxidation reaction.

It is not necessary that the entire mass or bed of filler comprise asinterable or self-bonding filler or contain a sintering or bondingagent, although such arrangement is within the purview of the invention.The self-bonding filler and/or the bonding or sintering agent may bedispersed only in that portion of the bed or filler adjacent to andsurrounding the mold of parent metal to a depth sufficient to form uponsintering or otherwise bonding an encasement of the developing cavitywhich is of sufficient thickness and mechanical strength to preventcollapse of the cavity (and consequent loss of fidelity of its shape inthe grown ceramic body to the shape of the parent metal mold) before asufficient thickness of the oxidation reaction product is attained.Thus, it suffices if a "support zone" of filler enveloping the moldcomprises a filler which is inherently sinterable or self-bonding withinthe appropriate temperature range or contains a sintering or bondingagent which is effective within the appropriate temperature range. Asused herein and in the claims, a "support zone" of filler is thatthickness of filler enveloping the mold which, upon bonding, is at leastsufficient to provide the structural strength necessary to retain thereplicated geometry of the mold until the growing oxidation reactionproduct becomes self-supporting against cavity collapse as describedabove. The size of the support zone of filler will vary depending on thesize and configuration of the mold and the mechanical strength attainedby the sinterable or self-bonding filler in the support zone. Thesupport zone may extend from the surface of the mold into the filler bedfor a distance less than that to which the oxidation reaction productwill grow or for the full distance of growth. In fact, in some cases thesupport zone may be quite thin. For example, although the support zoneof filler may be a bed of filler encasing the mold and itself encasedwithin a larger bed of non-self-bonding or non-sinterable filler, thesupport zone may in suitable cases comprise only a coating ofself-bonding or sinterable particles adhered to the mold by a suitableadhesive or coating agent. An example of this coating technique is givenbelow.

In any case, the filler should not sinter, fuse or react in such a wayas to form an impermeable mass so as to block the infiltration of theoxidation reaction product therethrough or, when a vapor-phase oxidantis used, passage of such vapor-phase oxidant therethrough. Further, anysintered mass which does form should not form at such a low temperatureas to fracture due to the expansion mismatch between the metal and thefiller before the growth temperature is reached, creating anon-homogeneous composite during development of the matrix due to thematrix subsequently solely filling the fractures in the bonded filler.For example, aluminum parent metal undergoes not only thermal expansionupon heating of the solid or molten metal but a significant volumeincrease on melting. This requires that the bed of filler in which theparent metal mold is embedded not sinter or otherwise self-bond to forma rigid structure encasing the parent metal mold prior to differentialexpansion thereof with respect to the filler, lest the expansion crackthe self-bonded structure. If this occurs, the replicated shape of themold is lost or, more typically, a non-homogeneous composite developsupon infiltration of the fractured bed of filler by the growth ofoxidation reaction product from the parent metal.

As noted previously, a bonding or sintering agent may be included as acomponent of the filler in those cases where the filler would nototherwise have sufficient inherent self-bonding or sinteringcharacteristics to prevent collapse of the cavity being formed into thevolume formerly occupied by the mold. This bonding agent may bedispersed throughout the filler or in the support zone only. Suitablematerials for this purpose include organo-metallic materials which underthe oxidizing conditions required to form the oxidation reaction productwill at least partially decompose and bind the filler sufficiently toprovide the requisite mechanical strength. The binder should notinterfere with the oxidation reaction process or leave undesiredresidual by-products within the ceramic composite product. Binderssuitable for this purpose are well known in the art. For example,tetraethylorthosilicate is exemplary of suitable organo-metallicbinders, leaving behind at the oxidation reaction temperature a silicamoiety which effectively binds the filler with the requisite cohesivestrength.

In practicing the process of this invention, the set-up of the parentmetal and bed in an oxidizing environment is heated to a temperatureabove the melting point of the metal but below the melting point of theoxidation reaction product, resulting in a body or pool of molten metal.On contact with the oxidant, the molten metal will react to form a layerof oxidation reaction product. Upon continued exposure to the oxidizingenvironment, within an appropriate temperature region, the remainingmolten metal is progressively drawn into and through the oxidationreaction product in the direction of the oxidant and into the bed offiller and there, on contact with the oxidant, forms additionaloxidation reaction product. At least a portion of the oxidation reactionproduct is maintained in contact with and between the molten parentmetal and the oxidant so as to cause continued growth of thepolycrystalline oxidation reaction product in the bed of filler, therebyembedding filler within the polycrystalline oxidation reaction product.The polycrystalline matrix material continues to grow so long assuitable oxidation reaction conditions are maintained.

The process is continued until the oxidation reaction product hasinfiltrated and embedded the desired amount of filler. The resultingceramic composite product includes filler embedded by a ceramic matrixcomprising a polycrystalline oxidation reaction product and including,optionally, one or more non-oxidized constituents of the parent metal orvoids, or both. Typically in these polycrystalline ceramic matrices, theoxidation reaction product crystallites are interconnected in more thanone dimension, preferably in three dimensions, and the metal inclusionsor voids may be partially interconnected. When the process is notconducted beyond the exhaustion of the parent metal, the ceramiccomposite obtained is dense and essentially void-free. When the processis taken to completion, that is, when as much of the metal as possibleunder the process conditions has been oxidized, pores in the place ofthe interconnected metal will have formed in the ceramic composite. Theresulting ceramic composite product of this invention possessessubstantially the original dimensions and geometric configuration of theoriginal mold, adjusted for melting point and thermal expansiondifferential volume changes of the parent metal during processing withrespect to the composite body formed and cooled.

Referring now to the drawings, FIG. 1 shows a refractory vessel 2, suchas an alumina vessel, containing a bed of filler 4 within which isembedded a mold 6 of parent metal. As shown in FIGS. 1 and 2, mold 6 hasa center section 8 which is generally cylindrical in configuration andjoins a pair of end sections 8a, 8b which are axially shorter but ofgreater diameter than center section 8. Generally, mold 6 has adumb-bell like configuration comprising generally circular disc-shapedend sections joined by a smaller diameter center section.

Upon heating of the assembly of FIG. 1 to a sufficiently hightemperature to melt the metal, a vapor-phase oxidant, which permeatesthe bed of filler 4, and is in contact with the molten metal, oxidizesthe molten metal and growth of the oxidation reaction product resultingtherefrom infiltrates the surrounding bed of filler 4. For example, whenthe parent metal is an aluminum parent metal and air is the oxidant, theoxidation reaction temperature may be from about 850° C. to about 1450°C., preferably from about 900° C. to about 1350° C., and the oxidationreaction product is typically alpha-alumina. The molten metal migratesthrough the forming skin of oxidation reaction product from the volumeformerly occupied by mold 6, which will result in a lowered pressurewithin that volume due to impermeability to the surrounding atmosphereof the growing skin of oxidation reaction product and a net pressureacting on the container-like skin of oxidation reaction product.However, the bed of filler 4 (or a support zone thereof) enveloping mold6 is intrinsically self-bonding at or above a self-bonding temperaturewhich lies above the melting point of the parent metal and close to butbelow the oxidation reaction temperature. Thus, upon being heated to itsself-bonding temperature, but not before, the filler, or a support zonethereof, has sintered or otherwise bonded to itself and attached to thegrowing oxidation reaction product sufficiently to afford sufficientstrength to the filler surrounding the developing cavity, i.e., thesupport zone of filler, to resist the pressure differential and therebyretain within the bed of filler 4 the geometry of the filled cavityformed therein by conformance of the filler to the shape of mold 6. Asdescribed in detail above, if the filler were to self-bond significantlyprior to completion of expansion of the parent metal upon heating andmelting thereof, the self-bonded filler would be cracked or broken byexpansion of the metal. In an embodiment in which only a support zone offiller 4 contains or comprises a sinterable or self-bonding filler or abonding or sintering agent, dotted line 5 in FIG. 1 indicates the extentof the support zone in the bed of filler 4. As the reaction continues,the cavity within bed 4 formerly filled by mold 6 is substantiallyentirely evacuated by the migration of molten parent metal through theoxidation reaction product to the outer surface thereof where itcontacts the vapor-phase oxidant and is oxidized to form additionaloxidation reaction product. The oxidation reaction product comprises apolycrystalline ceramic material which may contain inclusions therein ofunoxidized constituents of the molten parent metal. Upon completion ofthe reaction and evacuation of the volume formerly occupied by mold 6,the assembly is allowed to cool and the resultant ceramic composite,whose dimensions are indicated by dotted line 7 in FIG. 1 is separatedfrom excess filler, if any, left within vessel 2. Such excess filler orpart thereof may form a coherent body because of the sintering orself-bonding and may readily be removed from the ceramic composite whichit encases by gritblasting, grinding, or the like. An economicaltechnique is to employ grit blasting utilizing grit particles of amaterial which is suitable as the filler or as a component of the fillerso that the removed filler and grit may be reused as filler in asubsequent operation. It is important to recognize that the degree ofstrength of the self-bonded filler necessary to prevent cavity collapseduring processing is typically much less than the strength of theresulting composite. Hence, it is in fact quite feasible to removeexcess self-bonded filler by grit blasting without significant concernfor damaging the resultant composite. In any case, the ceramic compositestructure having the cavity formed therein may be further shaped bymachining or grinding or otherwise forming it to a desired outer shape.For example, as illustrated in FIG. 3, the ceramic composite 10 has beenground into the shape of a circular cylinder having an outer surface 12,opposite end faces 14a, 14b and having therein a cavity 16 which isdefined by surfaces congruent to the surfaces of mold 6. Thus, the shapeof cavity 16 is an inverse replication of the shape of mold 6, cavity 16being defined by end sections 18a, 18b and a connecting center section18 of lesser diameter than end sections 18a, 18b. For many applicationsthe ceramic body may be utilizable as formed following removal of theexcess, unentrained filler, without further requirement for grinding ormachining.

By selecting an appropriate filler and maintaining the oxidationreaction conditions for a time sufficient to evacuate substantially allthe molten parent metal from the filled cavity initially occupied bymold 6, a faithful inverse replication of the geometry of mold 6 isattained by cavity 16. While the illustrated shape of mold 6 (andtherefore of cavity 16) is relatively simple, cavities can be formedwithin the ceramic composite which inversely replicate with fidelity theshapes of molds of much more complex geometry than that of mold 6, bythe practices of the present invention. The outer surfaces of theceramic composite 10 may, if desired, be ground and machined orotherwise formed to any desired size or shape consistent with the sizeand shape of the cavity 16 formed therein.

It should be understood that the filler properties of being permeable,conformable, and self-bonding as described above are properties of theoverall composition of the filler, and that individual components of thefiller need not have any or all of these characteristics. Thus, thefiller may comprise either a single material, a mixture of particles ofthe same material but of different mesh size, or mixtures of two or morematerials. In the latter case, some components of the filler may, forexample, not be sufficiently self-bonding or sinterable at the oxidationreaction temperature but the filler of which it is a component part willhave the requisite self-bonding or sintering characteristics at andabove its self-bonding temperature because of the presence of othermaterials. A large number of materials which make useful fillers in theceramic composite by imparting desired qualities to the composite alsowill have the permeable, conformable and self-bonding qualitiesdescribed above. Such suitable materials will remain unsintered orunbonded sufficiently at temperatures below the oxidation reactiontemperature so that the filler in which the mold is embedded canaccommodate thermal expansion and melting point volume change, and yetwill sinter or otherwise self-bond only upon attaining a self-bondingtemperature which lies above the parent metal melting point but close toand below the oxidation reaction temperature, sufficiently to impart therequisite mechanical strength to prevent collapse of the forming cavityduring the initial stages of growth or development of the oxidationreaction product.

With respect to individual components of the filler, one suitable classof filler component includes those chemical species which, under thetemperature and oxidizing conditions of the process, are not volatile,are thermodynamically stable and do not react with or dissolveexcessively in the molten parent metal. Numerous materials are known tothose skilled to the art as meeting such criteria in the case wherealuminum parent metal and air or oxygen as the oxidant is employed. Suchmaterials include the single-metal oxides of: aluminum, Al₂ O₃ ; cerium,CeO₂ ; hafnium, HfO₂ ; lanthanum, La₂ O₃ ; neodymium, Nd₂ O₃ ;praseodymium, various oxides; samarium, Sm₂ O₃ ; scandium, Sc₂ O₃ ;thorium, ThO₂ ; uranium, UO₂ ; yttrium, Y₂ O₃ ; and zirconium, ZrO₂. Inaddition, a large number of binary, ternary, and higher order metalliccompounds such as magnesium aluminate spinel, MgO.Al₂ O₃, are containedin this class of stable refractory compounds,

A second class of suitable filler components are those which are notintrinsically stable in the oxidizing and high temperature environmentof the preferred embodiment, but which, due to relatively slow kineticsof the degradation reactions, can be incorporated as a filler phasewithin the growing ceramic body. An example is silicon carbide. Thismaterial would oxidize completely under the conditions necessary tooxidize, for example, aluminum with oxygen or air in accordance with theinvention were it not for a protective layer of silicon oxide formingand covering the silicon carbide particles to limit further oxidation ofthe silicon carbide. The protective silicon oxide layer also enablessilicon carbide particles to sinter or bond to themselves and to othercomponents of the filler under the oxidation reaction conditions of theprocess for aluminum parent metal with air or oxygen as the oxidant.

A third class of suitable filler components are those, such as carbonfibers, which are not, on thermodynamic or on kinetic grounds, expectedto survive the oxidizing environment or the exposure to molten aluminuminvolved with a preferred embodiment, but which can be made compatiblewith the process if 1) the environment is made less active, for example,through the use of CO/CO₂ as the oxidizing gases, or 2) through theapplication of a coating thereto, such as aluminum oxide, which makesthe species kinetically non-reactive in the oxidizing environment.

As a further embodiment of the invention and as explained in theCommonly Owned Patent and Applications, the addition of dopant materialsto the metal can favorably influence the oxidation reaction process. Thefunction or functions of the dopant material can depend upon a number offactors other than the dopant material itself. These factors include,for example, the particular parent metal, the end product desired, theparticular combination of dopants when two or more dopants are used, theuse of an externally applied dopant in combination with an alloyeddopant, the concentration of the dopant, the oxidizing environment, andthe process conditions.

The dopant or dopants (1) may be provided as alloying constituents ofthe parent metal, (2) may be applied to at least a portion of thesurface of the parent metal, or (3) may be applied to the filler or to apart of the filler bed, e.g., the support zone of the filler, or anycombination of two or more of techniques (1), (2) and (3) may beemployed. For example, an alloyed dopant may be used in combination withan externally applied dopant. In the case of technique (3), where adopant or dopants are applied to the filler, the application may beaccomplished in any suitable manner, such as by dispersing the dopantsthroughout part or the entire mass of filler as coatings or inparticulate form, preferably including at least a portion of the bed offiller adjacent the parent metal. Application of any of the dopants tothe filler may also be accomplished by applying a layer of one or moredopant materials to and within the bed, including any of its internalopenings, interstices, passageways, intervening spaces, or the like,that render it permeable. A convenient manner of applying any of thedopant material is to merely soak the entire bed in a liquid (e.g., asolution), of dopant material. A source of the dopant may also beprovided by placing a rigid body of dopant in contact with and betweenat least a portion of the parent metal surface and the filler bed. Forexample, a thin sheet of silicon-containing glass (useful as a dopantfor the oxidation of an aluminum parent metal) can be placed upon asurface of the parent metal. When the aluminum parent metal (which maybe internally doped with Mg) overlaid with the silicon-containingmaterial is melted in an oxidizing environment (e.g., in the case ofaluminum in air, between about 850° C. to about 1450° C., preferablyabout 900° C. to about 1350° C.), growth of the polycrystalline ceramicmaterial into the permeable bed occurs. In the case where the dopant isexternally applied to at least a portion of the surface of the parentmetal, the polycrystalline oxide structure generally grows within thepermeable filler substantially beyond the dopant layer (i.e., to beyondthe depth of the applied dopant layer). In any case, one or more of thedopants may be externally applied to the parent metal surface and/or tothe permeable bed. Additionally, dopants alloyed within the parent metaland/or externally applied to the parent metal may be augmented bydopant(s) applied to the filler bed. Thus, any concentrationdeficiencies of the dopants alloyed within the parent metal and/ orexternally applied to the parent metal may be augmented by additionalconcentration of the respective dopant(s) applied to the bed, and viceversa.

Useful dopants for an aluminum parent metal, particularly with air asthe oxidant, include, for example, magnesium metal and zinc metal, incombination with each other or in combination with other dopants asdescribed below. These metals, or a suitable source of the metals, maybe alloyed into the aluminum-based parent metal at concentrations foreach of between about 0.1-10% by weight based on the total weight of theresulting doped metal. The concentration for any one dopant will dependon such factors as the combination of dopants and the processtemperature. Concentrations within this range appear to initiate theceramic growth, enhance metal transport and favorably influence thegrowth morphology of the resulting oxidation reaction product.

Other dopants which are effective in promoting polycrystalline oxidationreaction growth, for aluminum-based parent metal systems are, forexample, silicon, germanium, tin and lead, especially when used incombination with magnesium or zinc. One or more of these other dopants,or a suitable source of them, is alloyed into the aluminum parent metalsystem at concentrations for each of from about 0.5 to about 15% byweight of the total alloy; however, more desirable growth kinetics andgrowth morphology are obtained with dopant concentrations in the rangeof from about 1-10% by weight of the total parent metal alloy. Lead as adopant is generally alloyed into the aluminum-based parent metal at atemperature of at least 1000° C. so as to make allowances for its lowsolubility in aluminum; however, the addition of other alloyingcomponents, such as tin, will generally increase the solubility of leadand allow the alloying material to be added at a lower temperature.

One or more dopants may be used depending upon the circumstances, asexplained above. For example, in the case of an aluminum parent metaland with air as the oxidant, particularly useful combinations of dopantsinclude (a) magnesium and silicon or (b) magnesium, zinc and silicon. Insuch examples, a preferred magnesium concentration falls within therange of from about 0.1 to about 3% by weight; for zinc in the range offrom about 1 to about 6% by weight; and for silicon in the range of fromabout 1 to about 10% by weight.

Additional examples of dopant materials useful with an aluminum parentmetal, include sodium, lithium, calcium, boron, phosphorus and yttriumwhich may be used individually or in combination with one or moredopants depending on the oxidant and process conditions. Sodium andlithium may be used in very small amounts in the parts per millionrange, typically about 100-200 parts per million, and each may be usedalone or together, or in combination with other dopant(s). Rare earthelements such as cerium, lanthanum, praseodymium, neodymium and samariumare also useful dopants, and herein again especially when used incombination with other dopants.

As noted above, it is not necessary to alloy any dopant material intothe parent metal. For example, selectively applying one or more dopantmaterials in a thin layer to either all, or a portion of, the surface ofthe parent metal enables local ceramic growth from the parent metalsurface or portions thereof and lends itself to growth of thepolycrystalline ceramic material into the permeable filler in selectedareas. Thus, growth of the polycrystalline ceramic material into thepermeable bed can be controlled by the localized placement of the dopantmaterial upon the parent metal surface. The applied coating or layer ofdopant is thin relative to the thickness of the parent metal body, andgrowth or formation of the oxidation reaction product into the permeablebed extends to substantially beyond the dopant layer, i.e., to beyondthe depth of the applied dopant layer. Such layer of dopant material maybe applied by painting, dipping, silk screening, evaporating, orotherwise applying the dopant material in liquid or paste form, or bysputtering, or by simply depositing a layer of a solid particulatedopant or a solid thin sheet or film of dopant onto the surface of theparent metal. The dopant material may, but need not, include eitherorganic or inorganic binders, vehicles, solvents, and/or thickeners.More preferably, the dopant materials are applied as powders to thesurface of the parent metal or dispersed through at least a portion ofthe filler. One particularly preferred method of applying the dopants tothe parent metal surface is to utilize a liquid suspension of thedopants in a water/organic binder mixture sprayed onto a parent metalsurface in order to obtain an adherent coating which facilitateshandling of the doped parent metal prior to processing.

The dopant materials when used externally are usually applied to aportion of a surface of the parent metal as a uniform coating thereon.The quantity of dopant is effective over a wide range relative to theamount of parent metal to which it is applied and, in the case ofaluminum, experiments have failed to identify either upper or loweroperable limits. For example, when utilizing silicon in the form ofsilicon dioxide externally applied as the dopant for an aluminum-basedparent metal using air or oxygen as the oxidant, quantities as low as0.0001 gram of silicon per gram of parent metal together with a seconddopant having a source of magnesium and/or zinc produce thepolycrystalline ceramic growth phenomenon. It also has been found that aceramic structure is achievable from an aluminum-based parent metalusing air or oxygen as the oxidant by using MgO as the dopant in anamount greater than 0.0005 gram of dopant per gram of parent metal to beoxidized and greater than 0.005 gram of dopant per square centimeter ofparent metal surface upon which the MgO is applied. It appears that tosome degree an increase in the quantity of dopant materials willdecrease the reaction time necessary to produce the ceramic composite,but this will depend upon such factors as type of dopant, the parentmetal and the reaction conditions.

Where the parent metal is aluminum internally doped with magnesium andthe oxidizing medium is air or oxygen, it has been observed thatmagnesium is at least partially oxidized out of the alloy attemperatures of from about 820° to 950° C. In such instances ofmagnesium-doped systems, the magnesium forms s masgesium oxide and/ormagnesium aluminate spinel phase at the surface or the molten aluminumalloy and during the growth process such magnesium compounds remainprimarily at the initial oxide surface of the parent metal alloy (i.e.,the "initiation surface") in the growing ceramic structure. Thus, insuch is magnesium-doped systems, an aluminum oxide-based structure isproduced apart from the relatively thin layer of magnesium aluminatespinel at the initiation surface. Where desired, this initiation surfacecan be readily removed as by grinding, machining, polishing or gritblasting.

The ceramic composite structures obtained by the practice of the presentinvention will usually be a dense, coherent mass wherein between about5% and about 98% by volume of the total volume of the compositestructure is comprised of one or mere of the filler components embeddedwithin a polycrystalline ceramic matrix. The polycrystalline ceramicmatrix is usually comprised of, when the parent metal is aluminum andair or oxygen is the oxidant, about 60% to about 99% by weight (of theweight of polycrystalline matrix) of interconnected alpha-alumina andabout 1% to 40% by weight (same basis) of non-oxidized metallicconstituents, such as from the parent metal.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

To illustrate the replication of a complex geometry in a ceramic matrixcomposite containing silicon carbide particles, a 1 inch diameter by 6inch long threaded rod of aluminum containing 10% silicon and 3%magnesium was completely submerged into a bed of silicon carbide (NortonCo. 39 Crystalon, 90 mesh) and heated to a process setpoint temperatureof 1125° C. for 72 hours in air. The total furnace time equaled 87 hourswith 5 hour heat-up and 10 hour cool-down cycles.

The resulting composite material was cross sectioned to show thereplication of the threaded rod into the alumina ceramic matrix/siliconcarbide composite material, and as such is pictured in FIG. 4. Thecomposition of the resulting composite was confirmed by X-ray powderdiffraction analysis.

In this instance, self-bonding of the bed was observed and is believedto be a consequence of partial oxidation of the silicon carbideparticles at the process temperature to form a layer of silica bondingmaterial.

EXAMPLE 2

To illustrate the replication of a complex geometry in a ceramic matrixcomposite containing alumina particles, a layer of silicon dioxideparticles was applied with an organic binder to the surface of a 2 inchthreaded rod of aluminum containing 10% silicon and 3% magnesium. Therod was then completely embedded into a bed of alumina (Norton Co. 38Alundum, 220 mesh) and heated to a process setpoint temperature of 1250°C. for 50 hours. The total furnace time was 60 hours, with 5 hourheat-up and 5 hour cool-down cycles.

The cross sectioned rod showed the replication of the threaded rod, andas such is pictured in FIG. 5. The composition of the resultingcomposite material was confirmed by X-ray powder diffraction analysis.In this case, it is believed that the layer applied to the alloy surfacebonded to each other and to the adjacent alumina particles to form a"support zone" to enable the surface replication process.

It should be noted that the screw thread geometry of FIGS. 4 and 5 wouldbe particularly difficult to make by any of the traditional ceramicprocessing methods, but is quite readily produced by the process of thepresent invention.

EXAMPLE 3

This specific embodiment of the present invention illustrates theformation of a complex geometry in a ceramic matrix composite using abed of alumina particles in a bonding agent and a support zone. In thisexperiment, a 22 gauge stainless steel cylinder was used as thecontainer for the set-up of parent metal and filler. The container hadan internal diameter of 31/4 inches and 0.0625 inch diameter perforationto provide for a 40% open area for diffusion of a vapor-phase oxidantinto the bed of filler. This container was lined with a stainless steelscreen having 0.016 inch diameter holes and 30% open area to prevent theescape of filler material through the perforations of the container.This container or vessel was sealed at the bottom end with a stainlesssteel cap, and was filled with a pre-fired heterogeneous filler materialcomprised of 95 weight percent alpha-alumina particles (Norton Co. 38Alundum, 90 mesh) and 5 weight percent silicon dioxide (predominantly100 mesh size or larger). A rod of aluminum, measuring 26 inches long by11/16 inches in diameter and alloyed with 10% silicon and 3% magnesium,was cast as to have on its surface, over the center two thirds of itslength, 16 fin-like protrusions, which was used to demonstrate thefidelity of shape replication of a more complex mold. The rod wascovered uniformly over its entire surface with silicon dioxide(predominantly 100 mesh size or larger) applied thereto with an organicbinder. The rod was submerged into the above-described filler containedin the vessel such that growth of the ceramic matrix would be symmetricand axially toward the walls of the stainless steel vessel.

The system above was heated to a setpoint temperature of 1250° C. for225 hours. The total furnace time was 265 hours, with 10 hour heat-upand 30 hour cool-down cycles.

The above process produced a cohesive composite material having analpha-alumina matrix embedding the alphaalumina particles of the fillermaterial, as evidenced by X-ray powder diffraction analysis. The cavityexhibited high fidelity, inverse replication of the geometry of the castaluminum rod.

Although only a few exemplary embodiments of the invention have beendescribed in detail above, those skilled in the art will readilyappreciate that the present invention embraces many combinations andvariations other than those exemplified.

What is claimed is:
 1. A method for producing a self-supporting ceramiccomposite body comprising (1) a ceramic matrix obtained by oxidation ofat least one parent metal to form a polycrystalline material comprisingat least one oxidation reaction product of said at least one parentmetal with at least one liquid-phase oxidant; and (2) at least onefiller embedded by said ceramic matrix, the method comprising the stepsof:(a) positioning said at least one parent metal adjacent to at leastone permeable mass of said at least one filler and orienting said atleast one parent metal and said at least one filler relative to eachother so that formation of said at least one oxidation reaction productwill occur in a direction towards and into said at least one permeablemass of said at least one filler; (b) heating said at least one parentmetal to a temperature above its melting point but below the meltingpoint of its at least one oxidation reaction to form at least one bodyof molten parent metal and reacting the at least one molten parent metalwith an oxidant consisting essentially of said at least one liquid-phaseoxidant at said temperature to form said at least one oxidation reactionproduct, and at said temperature maintaining at least a portion of saidat least one oxidation reaction product in contact with and extendingbetween said at least one body of molten metal and said at least oneliquid-phase oxidant, to draw molten metal through the at least oneoxidation reaction product towards the at least one liquid-phase oxidantand towards and into the adjacent mass of the at least one filler sothat fresh oxidation reaction product continues to form within the massof at least one filler at an interface between the at least oneliquid-phase oxidant and previously formed the at least one oxidationreaction product; and (c) continuing said reacting for a time sufficientto embed at least a portion of the at least one filler within saidpolycrystalline material.
 2. A method for producing a self-supportingceramic composite body comprising (1) a ceramic matrix obtained byoxidation of at least one parent metal to form a polycrystallinematerial comprising at least one oxidation reaction product of said atleast one parent metal with at least one solid-phase oxidant; and (2) atleast one filler embedded by said matrix, the method comprising thesteps of:(a) positioning said at least one parent metal adjacent to atleast one permeable mass of the at least one filler and orienting saidat least one parent metal and said at least one filler relative to eachother so that formation of said at least one oxidation reaction productwill occur in a direction towards and into said at least one permeablemass of the at least one filler; (b) heating said at least one parentmetal to a temperature above its melting point but below the meltingpoint of its at least one oxidation reaction to form at least one bodyof molten parent metal and reacting the molten parent metal with anoxidant consisting essentially of said at least one solid-phase oxidantat said temperature to form said at least one oxidation reactionproduct, and at said temperature maintaining at least a portion of saidat least one oxidation reaction product in contact with and extendingbetween said at least one body of molten metal and said at least onesolid-phase oxidant, to draw molten metal through the at least oneoxidation reaction product towards the at least one solid-phase oxidantand towards and into the adjacent mass of the at least one filler sothat fresh oxidation reaction product continues to form within the massof the at least one filler at an interface between the at least onesolid-phase oxidant and previously formed oxidation reaction product;and (c) continuing said reacting for a time sufficient to embed at leasta portion of the at least one filler within said polycrystallinematerial.
 3. The method of claim 1, wherein said at least one parentmetal comprises aluminum, silicon, titanium, tin, zinc or hafnium. 4.The method of claim 1, wherein said at least one liquid-phase oxidantcomprises at least one of oxygen, nitrogen, a halogen, sulfur,phosphorus, arsenic, carbon, boron, selenium, tellurium, compoundsthereof or combinations thereof.
 5. The method of claim 1, wherein saidat least one liquid-phase oxidant is provided to at least a portion ofsaid at least one filler by immersing at least a portion of said atleast one filler in said liquid-phase oxidant.
 6. The method of claim 1,wherein said at least one liquid-phase oxidant is provided to at least aportion of said at least one filler as at least one solid precursor. 7.The method of claim 1, wherein said at least one liquid-phase oxidant isprovided to at least a portion said at least one filler as at least oneliquid precursor.
 8. The method of claim 6, wherein said at least onesolid precursor comprises at least one salt.
 9. The method of claim 7,wherein said at least one liquid precursor comprises at least onesolution of at least one material which is used to impregnate at least aportion of said at least one filler.
 10. The method of claim 6, whereinsaid at least one solid precursor melts under the oxidation reactionconditions to provide a suitable oxidant moiety.
 11. The method of claim7, wherein said at least one liquid precursor decomposes under theoxidation reaction conditions to provide a suitable oxidant moiety. 12.The method of claim 3, wherein said at least one solid-phase oxidantcomprises at least one of oxygen, nitrogen, a halogen, sulfur,phosphorus, arsenic, carbon, boron, selenium, tellurium, compoundsthereof or combinations thereof.
 13. The method of claim 3, wherein saidat least one parent metal comprises at least one of aluminum, silicon,titanium, tin, zirconium or hafnium.
 14. The method of claim 3, whereinsaid at least one solid-phase oxidant is dispersed throughout at least aportion of said at least one filler.
 15. The method of claim 3, whereinsaid at least one solid-phase oxidant comprises at least one coating onat least a portion of said at least one filler.
 16. The method of claim3, wherein said at least one solid-phase oxidant comprises at least onereducible compound.
 17. The method of claim 16, wherein said at leastone reducible compound comprises silicon dioxides or borides.
 18. Themethod of claim 2, further comprising providing at least one relativelyinert filler in combination with said at least one filler, wherein saidat least one relatively inert filler absorbs the heat of reaction ofsaid at least one parent metal and said at least one solid-phaseoxidant, thereby minimizing any thermal runaway.
 19. The method of claim5, wherein said at least one liquid-phase oxidant is provided tosubstantially all of said at least one filler.
 20. The method of claim6, wherein said at least one solid-phase oxidant is provided tosubstantially all of said at least one filler.
 21. A method forproducing a self-supporting ceramic composite body comprising (1) aceramic matrix obtained by oxidation of at least one parent metal toform a polycrystalline material comprising at least one oxidationreaction product of said at least one parent metal with an oxidant; and(2) at least one filler embedded by said matrix, the method comprisingthe steps of:(a) positioning said at least one parent metal adjacent toat least one permeable mass of the at least one filler and orientingsaid at least one parent metal and said at least one filler relative toeach other so that formation of said at least one oxidation reactionproduct will occur in a direction towards and into said at least onepermeable mass of the at least one filler; (b) heating said at least oneparent metal to a temperature above its melting point but below themelting point of its at least one oxidation reaction to form at leastone body of molten parent metal and reacting the molten parent metalwith an oxidant consisting essentially of at least one solid-phaseoxidant and at least one liquid-phase oxidant at said temperature toform said at least one oxidation reaction product, and at saidtemperature maintaining at least a portion of said at least oneoxidation reaction product in contact with and extending between said atleast one body of molten metal and said oxidant, to draw molten metalthrough the at least one oxidation reaction product towards the oxidantand towards and into the adjacent mass of the at least one filler sothat fresh oxidation reaction product continues to form within the massof the at least one filler at an interface between the oxidant andpreviously formed oxidation reaction product; and (c) continuing saidreacting for a time sufficient to embed at least a portion of the atleast one filler within said polycrystalline material.